Radiography screens are well known and take various forms. As an example, U.S. Pat. No. 3,872,309 discloses a sheet of a high atomic number (high-Z) metal and a sheet of a fluorescent material bonded together onto a suitable substrate. The high-Z metal sheet is effective to absorb incident radiation photons, such as X-rays or gamma rays. The patent cites, as an example, X-rays of 60 keV energy level. Absorption of the radiation results in the emission of primary electrons and secondary electrons. The secondary electrons which are emitted in the direction of the adjacent sheet of fluorescent material cause the fluorescent material to emit photons of light which can be used to expose photographic film to provide a permanent record of the radiographic image. As described in the patent, the particular fluorescent material is chosen such that its light emission wavelength is optimally matched to the spectral sensitivity of the silver halide emulsion.
In another example disclosed in U.S. Pat. No. 4,256,965, a high energy fluoroscopic screen is described that employs a high-Z foil to convert incident high energy X-ray photons into secondary radiation that, in turn, causes a contiguous phosphor layer to luminesce and emit light which is gathered by a polycellular image converter. The light from the image converter is then sensed by a low light level TV camera/monitor chain for direct viewing of the image.
Such screens are relatively effective in converting high energy incident photon radiation, such as X-rays and gamma rays, into visible light emissions or light emissions that spectrally matched to film emulsions for recording. However, because of the geometry of the screens using separate, contiguous layers of high-Z and luminescent materials, at least half of the conversion efficiency is lost by virtue of the fact that only the forward secondary emissions from the high-Z layer reach the luminescent layer.
It is therefore an object of the invention to provide a radiographic imaging screen that converts a high percentage of incident radiation photons into light.
It is a further object of the invention to provide a radiographic imaging screen that effectively converts secondary electron emissions resulting from radiation absorption into visible light irrespective of the direction of travel of the emitted secondary electrons.
It is a further object of the invention to provide a radiographic imaging screen having improved efficiency of conversion of absorbed radiation photons into visible light photons.